1. Field of the Invention
The present invention relates to a method of manufacturing a photomask, and more particularly, to a method of manufacturing a photomask by which the photomask is corrected for minimizing manufacturing variations caused by the mask pattern density.
2. Description of the Background Art
With the recent miniaturization and higher integration of semiconductor devices, a pattern transferred to a substrate (hereinafter also referred to as a transferred pattern) using a photomask has dimensions substantially the same as or less than the wavelength of exposure light, raising the problem of optical proximity effect. This optical proximity effect is the interference effect of a light radiation energy caused by a mask pattern itself or an adjacent mask pattern thereto at the time of exposure. This optical proximity effect causes a transferred pattern to be shifted from a photomask pattern. That is, this effect causes degraded size controllability in a photolithography process. Conventionally, optical proximity correction has been performed on photomasks for improving the size controllability in the photolithography process. The optical proximity correction is a method of correcting the configuration of a mask pattern of a photomask by previously taking into consideration the shift of a transferred pattern from a photomask pattern caused by the optical proximity effect.
Another correction method is to provide a dummy pattern for reducing a difference in density between patterns. Still another correction method is disclosed in Japanese Patent Application Laid-Open No. 10-326010 (1998) (Document 1) (pp. 5–12, FIGS. 1–12). Document 1 describes that pattern data of a plurality of photomasks is received at once, and the entire region of each photomask is subjected to a correction for the optical proximity effect in a photoresist. More specifically, an underlayer correction region is automatically extracted from the entire region of each photomask for making the optical proximity correction resulting from a base structure and material of the photoresist.
Such optical proximity correction, however, only corrects the mask pattern configuration of a photomask with regard to the optical proximity effect appearing in the range of the order of several micrometers. Thus, the conventional optical proximity correction does not correct the shift of a finished pattern after transfer and etching from a photomask pattern caused by the pattern density in the range of the order of several tens micrometers or greater. Correcting the shift of a transferred pattern from a photomask pattern caused by the pattern density in the range of the order of several tens micrometers or greater is hereinafter also referred to as pattern-density-induced correction.
Further, the correction of mask pattern configuration disclosed in Document 1 is directed to the optical proximity effect resulting from the base structure and material of the photoresist, but it is not directed to the shift of a transferred pattern from a photomask pattern caused by the pattern density in the range of the order of several tens micrometers or greater.